Computers and other electrical devices operate using printed circuit boards (PCB's), thin substrates on which chips or other electronic components are mounted. In the context of personal computers (PC's), some circuit boards, called backplanes, contain sockets for expansion cards, special circuit boards that, when inserted into the backplane, add new capabilities to the computer.
Backplanes are often described as active or passive. Active backplanes contain logical circuitry that performs computing functions. On the other hand, passive backplanes contain almost no computing circuitry. Most backplanes used in personal computers are active, but there has been a recent move toward passive backplanes.
In a passive backplane system, active components such as the CPU are inserted on an additional card, making it easier to upgrade and to repair faulty components. Whether a backplane is active or passive, a PCB inserted into an expansion slot can communicate with another PCB inserted in the backplane via the PCB's edge connector, a tabbed edge of the PCB containing a plurality of parallel traces. When inserted into an expansion slot, the traces on the edge connector connect with a corresponding plurality of traces inside the expansion slot. These internal traces connect through the backplane to other expansion slots and to other components on the backplane itself. In this manner, the backplane's internal bus architecture can be used to communicate data from one PCB to another PCB located further down the backplane.
Though effective, the internal bus approach is problematic. First, the large number of required traces and connectors quickly consumes available board space. Second, though the rate of data transfer is theoretically only limited by the clock speed of the bus, bottlenecks often cripple the rate of data transfer and impair communication between circuit boards. Third, inserting or removing a circuit board during operation of the computer or electronic device is almost unthinkable. At the very least, doing so may cause a minor data loss. At worst, a system crash may result. Consequently, it is difficult to diagnose, repair, and/or replace faulty expansion cards without first shutting down the entire system. Fourth, communication channels are only established when the expansion cards are properly seated within the expansion slots. Fifth, signal quality may be at risk if specific engineering guide lines are not followed such stripline or Micro-Strip. Gaps in data transmission may occur if the card is removed or is not properly seated.
FIG. 1 illustrates a common circuit board 100, which consists of chips 102, traces 103 and other components (104, 105) attached to a single or multi-layer substrate. Traces 103 terminate at edge connector 106, which is the part of the circuit board that is inserted into an expansion slot in a backplane. Though most expansion cards use copper traces, there has been a recent move towards replacing the copper traces with a single optical fiber. Wave division multiplexing gives a single optical fiber tremendous bandwidth, but optical fiber suffers from the same problems affecting copper traces. For example, PCB's using optical fiber must be properly seated within an expansion slot to work properly, and should not be inserted or removed without first shutting down the entire system.
Today's high availability systems operate continuously around the clock. Consequently, new developments in fault-tolerant technology are required. Such developments should virtually eliminate the need to physically connect PCB's with copper traces or optical fiber, and should enable expansion cards to be removed or added to a system's backplane without disrupting system operation.
As will be evident from the figures and accompanying written descriptions, the open air communication channel embodied by the present invention supplies solutions to these and other needs long felt in the art.